1. Field of the Invention
The present invention relates to a method of forming a resist pattern and, more particularly, to a method of forming a resist micropattern on a substrate with high precision in a relatively short period of time.
2. Description of the Prior Art
A conventional simultaneous pattern transfer technique has been used as a means for forming a pattern of semiconductor ICs or the like on a substrate by using a mask. With this technique, a film of a resin material, i.e., a resist, sensitive (crosslinked or decomposed) to an energy beam is formed, and an energy beam carrying pattern information, such as light, irradiates (i.e., exposes) the resist film which is then developed to obtain a resist pattern. The semiconductor substrate is processed using the resultant resist pattern.
When the resist pattern is miniaturized, the resolution of the pattern to be formed is degraded due to the diffraction phenomenon. For this reason, high energy beams such as electron beams or X-rays are effectively used for fine lithography. However, even if such a high energy beam is used, the resolution of the resultant pattern is still low.
An example of a conventional method will be described wherein an electron beam is used as an energy beam and a positive electron beam resist material is used as a resist material to form a resist pattern. According to this method, the electron beam irradiates a positive electron beam resist film on a semiconductor substrate in an amount such that the resist film can respond to the beam, i.e., decomposed, along the direction of the entire thickness thereof. The electrons which enter into the resist film and the substrate collide with molecules and atoms thereof and are scattered back out of the substrate.
A scattering distance of these electrons generally falls within the range of several microns to several tens of microns although the distance varies depending on an energy level of the irradiated electron beam and a density of the substrate. During scattering of the electron beam, the positive resist material responds to the beam again. As a result, an area of the positive resist film decomposed by the beam is larger than an incident beam size (i.e., a beam profile) of the electron beam.
When the exposed resist film is developed under the above condition, a resist pattern has a size larger than that of the electron beam. In particular, when the pattern density is high, its resolution is greatly degraded by the back-scattered electrons generated when an adjacent pattern is irradiated with the electron beam.
The above situation also applies to the degradation of resolution due to the presence of the substrate such as the generation of secondary electrons in X-ray lithography leading to the degradation of resolution, or the generation of a standing wave in photolithography leading to pattern edge nonuniformity.
In addition, since an electron beam spot having a size smaller than the pattern size is used to scan within each pattern in the conventional method of forming a resist pattern with electron beams, a long scanning time is required.